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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var num = 1;
        function demo() {
            var num;
            console.log(num);//undefined
            function demoSon() {
                console.log(num);//2, demoSon();写在了 var num = 2后所以有值
                num = 3;//未声明,所以全局变量,不动
                console.log(num);//3
            }
            var num = 2//声明变量,值不动,变量到所在域的最前面
            demoSon();
        }
        demo();


    </script>
</head>

<body>

</body>

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